In a hydrogen infrastructure constitution business for building a hydrogen society, the spread of hydrogen stations for storing and supplying high-pressure hydrogen is important. In order to constitute the hydrogen stations having high reliability, development of high-pressure hydrogen gas pressure vessels is indispensable, and development of excellent materials for the pressure vessels has been desired. Here, metal materials, particularly steel materials, have promise as the materials for the pressure vessels, from the viewpoints of cost and recyclability.
As a technical trend, it has been desired that the pressure of stored gas is made higher in order to extend a travel distance of hydrogen cars, and it has been envisioned that the high-pressure hydrogen gas of 35 MPa or more is stored in the pressure vessels of the hydrogen stations. However, in carbon steels or high-strength low-alloy steels, it has been considered that hydrogen environment embrittlement occurs under a high-pressure hydrogen gas environment, and a steel material which can be used under a high-pressure hydrogen gas environment of 35 MPa or more has been almost limited to an austenitic stainless steel until now. The austenitic stainless steel is generally more expensive than a ferritic steel, and has a stable austenite phase up to room temperature, so that strength adjustment by heat treatment cannot be performed. Accordingly, as the material for the pressure vessels for storing the higher-pressure hydrogen gas, a high-strength ferritic steel represented by a Cr—Mo steel has been desired.
As a conventional technique, for example, patent literature 1 proposes a carbon steel or a low-alloy steel under a high-pressure hydrogen environment, a seamless steel pipe produced therefrom, and a method for producing the same. In this proposed technique, the Ca/S ratio of constituents is controlled, thereby decreasing the amount of diffusible hydrogen in the steel to improve high-pressure hydrogen environment embrittlement resistance characteristics.    Patent Literature 1: JP-A-2005-2386